Extending Dynamic Range with Contrast Masking

Tony Gondola, 6-07

The goal of contrast masking is to bring a wide tonal variations into a more manageable range. Darkroom workers of old used the techniques of dodging and burning to bring about a similar result. This is similar to what can be achieved using Luminosity Masking but without the need for exposure bracketing. As such, the technique can be applied to existing data for which bracketed AVI data doesn't exist.

To illustrate here is a typical single frame image taken near the terminator where tonal range is generally the greatest:

This image my seem a bit dark but it's actually as light as one could go with severely clipping the highlights in the rim of Archimedes. There is some clipping in the rim of the smaller crater Bancroft but this is typical for deep terminator images. The goal with contrast masking this image is to bring up the detail in the dark areas while leaving the highlights as is.

The

The first step in Photoshop (other image processing programs will be similar) is to bring up the Layers window. Right click on the background layer and select duplicate layer. Take the defalt values and you'll have a copy of the image in the layer stack. Select the copy and apply a Gaussian blur of anything from 5 to 15 pixels. In this case I've selected 10:

The details are blurred but the overall form of the major features are still visible.

Now, invert the layer to a negative:

Finally, go back to the layers box and change the blending mode from normal to overlay:

The overall effect is rather dramatic, compressing the tones and really opening up the image. Depending on the makeup of the original image the effect can sometimes be too strong. In this case I think it is so we'll moderate the effect by reducing the opacity, in this case down to 50%. Here's the result side by side with the original:

Overall the effect is rather impressive as there's a slight enhancement of detail along with control of the tonal range. The technique is simple and provides a good alternative to the use of curves and levels for tonal range adjustment.